Many applications have materials requirements that may be expensive, difficult, or impossible to meet in a single, monolithic material. For example, certain applications call for the chemical and/or thermal stability of ceramic materials, yet also require mechanical flexibility and amenability to certain types of processing (e.g., extrusion, spin-coating, etc.) found in some polymeric materials. In such instances, composite and/or hybrid materials that combine two or more different types of materials may be used. Examples span a variety of different fields and applications, including electrochemical batteries, aerospace engineering and armor.
Without limiting the scope of the application, one example of a hybrid material application is hybrid material used in battery separators or separators in electrochemical cells. A battery or electrochemical cell separator must separate electrodes from one another but may maintain a sufficient degree of ionic conductivity. The separator may be a thin, porous insulating material with good mechanical strength. Polymeric separators are often used for their high mechanical strength and amenability to processing techniques, such as those processing techniques that introduce a high degree of porosity. Examples of commonly used polymers in battery separators include organic polyolefin and composite materials, e.g., polypropylene, polyethylene, polypropylene, etc. Certain conventional polymeric materials, however, may lack thermal stability, chemical stability, or both. This may make them less than ideal for certain applications, such as exposure to chemically corrosive environment or high temperatures, one or both of which may occur in high-performance batteries. For this reason, high performance batteries tend to use inorganic separators (e.g., glass and ceramic separators) that are more compatible with their corrosive, non-polymeric electrolytes. Certain inorganic separators may have disadvantages, such as brittleness or challenges in machining process.
FIG. 1A illustrates a ceramic/polymer composite material 10 that may be used as a battery separator in the Prior Art. As illustrated in FIG. 1A, a substrate 2, typically polymeric, may be functionalized with an oxide group 4. The substrate 2 may be porous, which may increase the surface area for ion exchange. Surface functionalization 4 allows the coating of the substrate 2 with a layer of material 6 to improve properties such as thermal and chemical stability and wettability. Wettability by various electrolytes used in battery applications is of particular concern because separators with low electrolyte wettability can degrade or decrease the efficiency of the battery. In many examples, the material 6 may include particles 6a of a ceramic, glass and/or metal oxide composition. Such particles 6a may be deposited on the substrate 2 in a number of ways, such as by sol-gel processing or by wet deposition. Generally, in order to keep the material 6 intact, it is necessary to employ some kind of a binder 6C to bind the particles 6a to one another. To get the material 6 to fix or stay on the surface, it may also be necessary to heat the material at temperatures high enough to sinter the ceramic particles 6a. Sintering may cause chemical bonding in some of the particles 6a with other particles 6a or with binder 6C. For example, sintering may activate the chemical cross-links from particle 6a-binder 6C-particle 6a and/or particle 6a-binder 6C-substrate 2. In some cases, sintering may even cause particles 6a to fuse together partially or completely. However, the high heat of the sintering process may degrade the substrate 2. Moreover, the use of a binder 6C has significant disadvantages, including placing inherent limitations on the density of particles 6a in the material as well as introducing chemical agents into the composite material 10 that may be leech or degrade in the harsh chemical environment of an electrochemical cell.
FIG. 1B is an electron micrograph of an exemplary substrate. FIG. 1B is re-printed from S. S. Zhang, “A review on the separators of liquid electrolyte Li-ion batteries,” Journal of Power Sources, Volume 164, Issue 1, 2007, page 351. FIG. 1B shows a nonwoven fabric substrate 15a. Specifically, FIG. 1B illustrates a top view image of a nonwoven fabric substrate 15a prior to the addition of particles 6a. FIG. 1B illustrates the nonwoven fabric substrate 15a having several fibers 15b. 
FIG. 1C shows an electron micrograph top view of a ceramic/nonwoven separator 600 made by depositing a micro particle, metal oxide and binder coating 630 on a surface of a commercial substrate. FIG. 1C is re-printed from S. S. Zhang, “A review on the separators of liquid electrolyte Li-ion batteries,” Journal of Power Sources, Volume 164, Issue 1, 2007, page 351. FIG. 1C shows, for example, several of the metal oxide particles 614. A binder 616 (not visible in FIG. 1C) is also present between the oxide particles 614. FIG. 1C shows, for example, several of the metal oxide particles 614 as well as a binder 616 between the particles 614.
In each of these examples, the protective ceramic layer is relatively thick (e.g., on order of several microns). Moreover, in some applications or processes, the protective layer is applied using a binder and/or high temperature sintering that may degrade or limit the properties of the substrate, the protective layer or both. In either case, the chemical and thermal stability of the composite or hybrid material may be compromised.